Feedthrough capacitors and MLCC chip capacitors are well known in the prior art for active implantable medical devices (AIMDs). One is directed to U.S. Pat. Nos. 5,333,095; 5,905,627; 6,275,369; 6,529,103; and 6,765,780 all of which are incorporated herein by reference. The hermetic seal feedthrough terminal assemblies generally consist of a titanium ferrule into which an alumina hermetic seal is gold brazed. One or more lead wires penetrate through the alumina in non-conductive relationship with the ferrule. Gold brazes are also used to form a hermetic terminal between the one or more leadwires and the alumina ceramic.
First, some general information concerning good engineering design practice for electromagnetic interference (EMI) filters. It is very important to intercept the EMI at the point of lead conductor ingress and egress to the AIMD. It would be an inferior practice to put filtering elements down in the circuit board as this would draw EMI energy inside of the AIMD housing where it could re-radiate or cross-couple to sensitive AIMD circuits. A superior approach is to mount one or more feedthrough or MLCC-type capacitors right at the point of leadwire entrance so that it can be coupled to high frequency EMI signals from the lead conductors directly to the AIMD housing, which acts as an energy dissipating surface.
There are some interesting design challenges however. The titanium ferrule, which is laser welded into the overall AIMD housing, is at ground potential. Titanium tends to form oxides which act as either insulators or semi-conductors. Accordingly, grounding the feedthrough capacitor electrode plates directly to the titanium ferrule is contra-indicated. Reference is made to U.S. Pat. No. 6,465,779 (which is incorporated with this reference) which describes gold bond pad areas where the feedthrough capacitor external metallization can be directly connected to gold. The gold to which the feedthrough capacitor is directly connected is the braze material used to form the hermetic seal between the alumina and the titanium ferrule. As noted above, the hermetic seal is formed via a brazing process. By attaching the capacitor's ground plates to the gold, one can be assured that there will be no oxide that will increase the capacitor's equivalent series resistance (ESR) which can seriously degrade the capacitor's performance at high frequency. An undesirable aspect of using the gold braze for attachment is that gold is very expensive. Accordingly, there is a need for methods that provide a reliable low impedance ground path which are oxide resistant for grounding of AIMD filter capacitors. The present invention fulfills these needs and provides other related advantages.